A cold-rolled ferritic stainless steel sheet, which is economical and excellent in terms of corrosion resistance, is used in various applications such as building materials, transportation instruments, home electrical appliances, kitchen equipment, chemical plants, water storage tanks, and automobile parts, and the range of its application has been expanding in recent years. In order to be used in these applications, such a cold-rolled steel sheet is required to have not only sufficient corrosion resistance but also sufficient formability (such as elongation and average Lankford value (hereinafter, also referred to as “average r value”)) which allows steel to be formed into a desired shape, and is required to be excellent in terms of surface appearance quality before and after a forming work process.
Conventionally, it is known that irregularities called “ridging”, which has a concave-convex shape parallel to the rolling direction, and irregularities called “surface roughening (orange peel)”, which is caused by undulation of crystal grains, are generated in a cold-rolled ferritic stainless steel sheet in a forming work process. Since such kinds of irregularities deteriorate surface appearance quality, such kinds of irregularities are removed in a following polishing process, and it is thereby preferable that the amount of such kinds of irregularities generated be as small as possible to reduce the polishing load.
In addition, since a stainless steel sheet, which is excellent in terms of corrosion resistance, is used without being subjected to coating or painting in many cases, its own appearance is also important. Specifically, since visual surface appearance such as surface gloss and the clarity of a reflected image influence buying intention, it is important to improve visual surface appearance. It is known that the visual surface appearance of a product depends on the smoothness of a surface and the existence of surface defects. Since waviness called “roping”, which is parallel to the rolling direction, deteriorates the clarity of a reflected image, and since rolling-induced surface defects typified by, for example, a dent flaw called “oil pit”, which is occurred on the surface of sheet during cold rolling by lubricant that drawn into roll-bite, and flaws generated by the transfer of the polishing marks of work rolls cause white and cloudy surface appearance, there is a decrease in commercial value. Therefore, it is required to achieve a smooth surface so that the surface becomes as close as possible to a mirror surface by inhibiting the waviness and the surface defects from occurring.
In response to such requirements, Patent Literature 1 discloses a method for manufacturing a cold-rolled stainless steel sheet, the method including heating a steel slab having a chemical composition containing, by mass %, C: 0.01% to 0.03%, Si: 0.02% to 0.030%, Mn 0.45% to 1.0%, P: 0.05% or less, S: 0.01% or less, Al: 0.01% to 0.20%, N: 0.01% to 0.06%, Cr: 16.0% to 18.0%, and the balance being Fe and inevitable impurities, to a temperature higher than 1050° C., performing hot rolling on the heated steel slab with a finishing delivery temperature of 800° C. to 1000° C., performing pickling, then performing cold rolling, holding the cold-rolled steel sheet at a temperature of 800° C. to 950° C. for 20 seconds, and then cooling the held steel sheet at a cooling rate of 10° C./s or more. Patent Literature 1 states that, with this, it is possible to obtain a cold-rolled ferritic stainless steel sheet having a metallographic structure, in which the proportion of a ferrite phase to the whole metallographic structure is 80% to 97% in terms of area ratio, and in which the average grain diameter of a ferrite phase is 5 μm to 20 μm, and a good strength-elongation balance corresponding to a TS×El of 15000 MPa·% or more, and being capable of decreasing the amount of ridging generated in a forming process. That is, in the technique according to Patent Literature 1, long-time annealing performed on a hot-rolled steel sheet is omitted, and the conditions of cold-rolled-sheet annealing and cooling are specified. However, in the case of the technique disclosed in Patent Literature 1, since long-time hot-rolled-sheet annealing is omitted, cold rolling is performed on a hardened hot-rolled steel sheet, which results in a significant deterioration in manufacturability in a cold rolling process.
In addition, Patent Literature 2 discloses a technique for increasing ridging resistance in which the chemical composition contains, by mass %, C: 0.02% or less, Si: 0.70% or less, Mn: 0.50% or less, P: 0.04% or less, S: 0.01% or less, Al: 0.01% to 0.15%, N: 0.02% or less, Cr: 16% to 23%, Ni: 0.50% or less, Ti: 0.10% or less, Nb: 0.01% or less, and Zr: 0.20% to 0.80%, and in which the effect of preventing a grain diameter from increasing as a result of Zr fixing C and N in the form of precipitates is utilized in order to control the average grain diameter of ferrite grains after a finish annealing process to be 15 μm or less. However, although the grain diameter is successfully controlled to be 15 μm or less through the effect of Zr, since a certain amount of Zr is contained, there is a problem of an increase in manufacturing costs, and there is a problem of a significant deterioration in formability, in particular, elongation after fracture due to a significant increase in yield strength through the pinning effect of Zr-based carbonitrides, since precipitation of Zr-based carbonitrides is utilized for inhibiting an increase in grain diameter.
Moreover, Patent Literature 3 discloses a technique in which gloss is improved by decreasing the amount of oil drawn into a roll bite in order to inhibit oil pits from occurring and, at the same time, by suppressing the transfer of concave-convex patterns on the surfaces of rolls as a result of using hard low-surface-roughness work rolls in a cold rolling process. However, while there is a certain effect of removing rolling-induced surface defects, it is not possible to solve a problem of surface defects due to a raw material such as roping, ridging, and surface roughening, and there is an increase in roll operational costs due to an increase in polishing load.